I most frequently use online reservations, at least in preference to phone ones if given the option. Phone works too, though. When actually on the road and making a reservation for a night or two in advance often the phone is easier and more productive.
I have not had difficulties with the quoted prices being incorrect.
Site specific reservations are not generally of great importance to me personally, provided of course I get a site suitable for my equipment. I think particularly irksome are some parks that only do site-specific reservations and so more or less require me to come up with a site number for a park I've never seen before and know little about. Michigan and Minnesota state parks are offenders in this category; if one arrives after hours with no prior reservation, one needs to call from a hotline phone and talk to a central reservation office to make a reservation for that night with someone who usually has no real direct knowledge of the campground, trying to come up with a reasonable site assignment prior to entering the campground and seeing what looks appropriate. On the other hand, if the sites vary and there's reasonable information about them, it can be handy to be able to pick one's preferred site ahead of time.
